Silver halide photography usually involves the exposure of silver halide photographic element with light in order to form a latent image that is developed during photographic processing to form a visible image. Conventional silver halide grains, such as cubic or polymorphic silver halide, typically has some intrinsic sensitivity to light in the blue region of the spectrum. In order to sensitize the silver halide to other than the blue region, sensitizing dyes are used in the silver halide emulsion. Sensitizing dyes are chromophoric compounds (for example, cyanine dye compounds). Their usual function is to adsorb to the silver halide and to absorb light (usually other than blue light) and transfer that energy via an electron to the silver halide grain thus, rendering the silver halide sensitive to radiation of a wavelength other than the blue intrinsic sensitivity. However, sensitizing dyes can also be used to augment the sensitivity of silver halide in the blue region of the spectrum.
The use of low bulk iodide (&lt;6%), high tabularity (&gt;25) tabular emulsions in the blue sensitive layer of photographic films exhibits a number of advantages over conventional high iodide, three dimensional emulsion grains (such as cubic, octahedral, tetrahedral and polymorphic grains). Among these advantages are: better green and red layer optical sharpness (due to the more specular light scattering properties of tabular grains), the potential for larger green-onto and red-onto blue interimage for color correction, and less developer seasoning and better fixing properties (due to the lower bulk iodide content). A disadvantage is the inherently lower intrinsic blue light sensitivity of the tabular emulsion, which manifests itself in an inability to faithfully reproduce certain blue colors, most notably that of saturated deep blue sky.
One solution to the lack of intrinsic blue light sensitivity is to dye the emulsion with a combination of blue spectral sensitizing dyes covering the wavelength region 400 to 500 nm. For convenience these combinations are usually limited to a short wavelength dye (400 to 445 nm) and a longer dye (greater than 445 nm). A number of references have previously described the use of blue sensitizing combinations. For example, U.S. Pat. Nos. 4,439,520 and 4,425,425 describe supersensitizing dye combinations on tabular grains or combinations which can provide an absorption peak intermediate between the dyes.
Blue spectral dye combinations on morphologies other than tabular grains have also been taught. For example, U.S. Pat. No. 4,518,689 describes the use of at least one monomethine cyanine dye on inner latent image type octahedral silver halide emulsions to provide an increase in spectral sensitivity and maximum density. EP 0 314 104 describes the use of dyes on cubic emulsions for lithographic applications. Binary dye combinations (400 to &lt;445 nm and 445 to 490 nm) on silver chloride emulsions are also described in U.S. Pat. No. 4,469,785 to impart good color reproducibility.
However, the use of two blue sensitizing dyes can result in loss of speed compared to the use of one dye alone. Additionally, some dye combinations may result in more fog (that is, a minimum density appearing in the developed element even in areas of no light exposure).
It would be desirable then to provide a photographic element which contains a high tabularity tabular grain emulsion in the blue sensitive emulsion, which is sensitized by at least two spectral sensitizing dyes so as to have good sensitivity in the blue region of the spectrum and still retains good speed while exhibiting low fog.